HomeMy WebLinkAbout22-100630-Geotechnical (Soils) Report-02-10-2022-V1
August 24, 2021
Steve Davis
6124 Panorama Drive NW
Tacoma, WA 98422
(253) 606-5586
stevedavisexcavating@gmail.com
Geotechnical Engineering Report
Proposed Commercial Development
803 South 348th Street
Federal Way, Washington
PN: 2021049087
Doc ID: Davis.S348thSt.RG
INTRODUCTION
This Geotechnical Engineering Report summarizes our recent site observations, subsurface
explorations, laboratory testing and engineering analyses, and provides geotechnical
recommendations and design criteria for the proposed Bigfoot Java drive-thru coffee shop to be
constructed at 803 South 348th Street in Federal Way, Washington. The location of the site is shown
on the Site Location Map, Figure 1.
Our understanding of the project is based on our discussions with you; our previous work at
the subject site; our August 3, 2021 site visit; and our understanding of the City of Federal Way
development codes, and our experience in the area. We understand that you propose to construct a
drive thru coffee shop with a paved parking area at the site. We anticipate that the structure will be a
one- story, wood framed structure founded on conventional shallow footings. The proposed site plan
is included as the Site & Exploration Plan, Figure 2.
We understand that the City of Federal Way uses the 2016 version of the King County Surface
Water Design Manual (KCSWDM) with addendum to confirm subsurface conditions and to address
the feasibility of the onsite infiltration of stormwater runoff generated by the proposed development.
This Geotechnical Engineering Report addresses the feasibility of the site soils to support infiltration
BMPs and provides geotechnical design recommendations for the proposed commercial
development.
SCOPE
The purpose of our services was to evaluate the surface and subsurface conditions across the
site as a basis for providing geotechnical recommendations and design criteria for the subject site.
Specifically, the scope of services for this project included the following:
1. Reviewing the available geologic, hydrogeologic, and geotechnical data for the site area;
2. Observing subsurface conditions across the site by monitoring the excavation of three test
pits at select locations across the site;
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3. Providing geotechnical conclusions and recommendations regarding site grading activities
including; site preparation, subgrade preparation, fill placement criteria, suitability of on -
site soils for use as structural fill, temporary and permanent cut and fill slopes, drainage
and erosion control measures;
4. Providing recommendations and design criteria for stormwater runoff mitigation
measures, including but not limited to a cutoff or French drain; and
5. Providing recommendations for seismic design parameters, including 2018 IBC site class;
6. Providing geotechnical conclusions regarding foundations and floor slab support and
design criteria, including bearing capacity and subgrade modulus as appropriate;
7. Providing our opinion about the feasibility of onsite infiltration including a preliminary
design infiltration rate based on grain size data per the 2016 King County Stormwater
Design Manual, if applicable; and,
8. Preparing a written Geotechnical Engineering Report summarizing our site observations
and conclusions, and our geotechnical recommendations and design criteria, along with
the supporting data.
The above scope of work was summarized in our Proposal for Geotechnical Engineering Services
dated August 2, 2021. We received written authorization to proceed the same day.
SITE CONDITIONS
Surface Conditions
As stated, the subject site is located at 803 South 348th Street in Federal Way, Washington in
an area of existing commercial development. According to the King County iMap website, the site is
rectangular in shape, measures approximately 100 feet wide (east to west) by approximately 250 feet
long (north to south) and encompasses approximately 0.57 acres. The site is mostly undeveloped but
there is a large modular concrete retaining wall on portions of the south and west side of the parcel.
The site is bounded by undeveloped land to the west and south, a parking lot to the east, and South
348th Street to the North.
According to topographic information obtained from the King County iMap website and our
site observations, the ground surface of the site is relatively flat with a gentle slope to the southwest
at approximately 1 to 5 percent. Along the southeastern, southern, and western borders of the site
there is a block wall that extends down from the site. The wall varies in height from approximately 3
to 10 feet. The total topographic relief of the site is on the order of approximately 15 feet. The existing
site layout and topography are shown on the Site Vicinity Map, Figure 3.
Vegetation across the site generally consists of sparse unmaintained grasses with scattered
coniferous and deciduous trees along the perimeters of the site. No springs or seepage was observed
at the time of our site visits. No evidence of erosion, soil movement, landslide activity or deep-seated
slope instability was observed at the site or the adjacent areas at the time of our site visit.
Site Soils
The Natural Resource Conservation Service (NRCS) Web Soil Survey maps the site as being
underlain by Everett-Alderwood gravelly sandy loams (EwC) soils and Seattle Muck (Sk) soils. An
excerpt of the NRCS map for the site vicinity is attached as Figure 4. Descriptions of the mapped soils
are provided below.
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Everett-Alderwood gravelly sandy loams (EwC): The Everett-Alderwood soils are mapped as
underlying the northern portion of the site, are derived from basal till with some volcanic ash, form
on slopes of 6 to 15 percent, are considered to have a “slight to moderate” erosion hazard when
exposed, and are included in hydrologic soils group B.
Seattle Muck (Sk) soils: The Seattle Muck soils are mapped as underlying the southern portion
of the site, are derived from grassy organic material, form on slopes of 0 to 1 percent, are considered
to have a “slight” erosion hazard when exposed, and are included in hydrologic soils group B/D.
Site Geology
The Geologic Map of the Poverty Bay 7.5-minute Quadrangle, Washington, Derek B. Booth,
Howard H. Waldron, and Kathy G. Troost (2003) indicates the site is underlain by Recessional Outwash
Deposits (Qvr), with glacial till (Qgt) being mapped within the site vicinity. These glacial soils were
deposited during the Vashon Stade of the Fraser glaciation, approximately 12,000 to 15,000 years ago.
An excerpt of the above referenced geologic map is attached as Figure 5. Descriptions of the mapped
geologic units are provided below.
Recessional Outwash Deposits (Qvr): The recessional outwash soils typically consist of a
poorly to well sorted, lightly stratified mixture of sand and gravel that may contain localized deposits
of clay and silt that were deposited by meltwater streams emanating from the retreating ice mass.
glacial till (Qgt): Glacial till is also mapped in the vicinity of the site and generally underlays the
recessional outwash. Glacial till consists of a heterogeneous mixture of clay, silt, sand, and gravel that
was deposited at the base of the prehistoric continental ice mass and was subsequently overridden.
No areas of landslide deposits or mass wasting are noted on the referenced map on or within the
immediate vicinity of the site.
Subsurface Explorations
On August 3, 2021, we visited the site and monitored the excavation of three test pits to depths
of about 8 to 8¼ feet below the existing grade, logged the subsurface conditions encountered in each
test pit, and obtained representative soil samples. The test pits were excavated by a small track -
mounted excavator operated by you.
The specific number, locations, and depths of our explorations were selected by based on the
proposed configuration of the proposed development and were adjusted in the field based on
consideration for site access limitations, underground utilities, existing site conditions, and current
site usage. The soil densities presented on the test pit logs are based on the difficulty of excavation
and our experience. Representative soil samples obtained from the explorations were placed in
sealed plastic bags and taken to our laboratory for further examination and testing as deemed
necessary. Each test pit was backfilled with the excavated soils and bucket tamped, but not otherwise
compacted.
The subsurface explorations excavated as part of this evaluation indicate the subsurface
conditions at specific locations only, as actual subsurface conditions can vary across the site.
Furthermore, the nature and extent of such variation would not become evident until additional
explorations are performed or until construction activities have begun. Based on our experience and
extent of prior exploration in the area, it is our opinion that the soils encountered in the exploration
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are generally representative of the soils at the site. Table 1, below, summarizes the approximate
functional locations, surface elevations, and termination depths of our test pits.
TABLE 1:
APPROXIMATE LOCATIONS, ELEVATIONS, AND DEPTHS OF EXPLORATIONS
Test Pit
Number Functional Location
Surface
Elevation1
(feet)
Termination
Depth
(feet)
Termination
Elevation1
(feet)
TP-1
TP-2
TP-3
Northern portion of site
Central portion of site
Southern portion of site
227
226
225
8¼
8
8
218¾
218
217
Notes:
1 = Surface Elevations estimated by interpolating between contours provided on the King County GIS (NAVD 88)
The approximate locations and numbers of our test pits are shown on the attached Site &
Exploration Plan, Figure 2. The indicated locations were determined by taping or pacing from existing
site features and reference points; as such, the locations should only be considered as accurate as
implied by the measurement method. The soils encountered were visually classified in accordance
with the Unified Soil Classification System (USCS) and ASTM D2488. The USCS is included in Appendix
A as Figure A-1, while the descriptive logs of our test pits are included as Figures A-2 and A-3.
Subsurface Conditions
At the locations of our subsurface explorations we encountered fairly uniform subsurface
conditions that, in our opinion, generally differed from the mapped stratigraphy. Our test pits
encountered approximately 5¼ to 7½ feet of grey poorly graded gravel with some coarse sand in a
dense, moist condition. We were informed that these soils are fill placed during previous construction
activities. No compaction tests were provided to us, but the soil did appear compacted. Underlying the
fill, our test pits encountered grey to tan silty sandy gravel in a dense, moist condition. We interpret
these soils to be consistent with glacial till. These soils were encountered to the full depth explored.
Laboratory Testing
Geotechnical laboratory tests were performed on select samples retrieved from our
exploration to estimate index engineering properties of the soils encountered. Laboratory testing
included visual soil classification per ASTM D2488, moisture content determinations per ASTM D2216,
and grain size analysis per ASTM D6913.
Groundwater Conditions
At the locations of our test pits we did not encounter groundwater seepage within the depths
explored. However, we did observe iron-oxide staining/discoloration, otherwise known as mottling, at
approximately 5¼ to 7½ feet below existing ground surface. Mottling is generally indicative of a seasonal
or fluctuating groundwater surface, often associated with perched groundwater. Perched groundwater
table develops when the vertical infiltration of precipitation through a more permeable soil, is slowed
at depth by a deeper, less permeable soil type. We anticipate fluctuations in the local groundwater
levels will occur in response to precipitation patterns, off-site construction activities, and site
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utilization. Analysis or modeling of anticipated groundwater levels during construction is beyond the
scope of this report.
ENGINEERING CONCLUSIONS AND RECOMMENDATIONS
Based on the results of our data review, site reconnaissance, subsurface explorations and our
experience in the area, it is our opinion that the construction of the drive-thru coffee shop is feasible
from a geotechnical standpoint. Pertinent conclusions and geotechnical recommendations regarding
the design and construction of the proposed development are presented below.
Seismic Design
The site is located in the Puget Sound region of western Washington, which is seismically
active. Seismicity in this region is attributed primarily to the interaction between the Pacific, Juan de
Fuca and North American plates. The Juan de Fuca plate is subducting beneath the North American
plate at the Cascadia Subduction Zone (CSZ). This produces both intercrustal (between plates) and
intracrustal (within a plate) earthquakes. In the following sections we discuss the design criteria and
potential hazards associated with the regional seismicity.
Seismic Site Class
Based on our observations and the subsurface units mapped at the site, we interpret the
structural site conditions to correspond to a seismic Site Class “C” for the onsite soils in accordance
with the 2018 IBC (International Building Code) documents and ASCE 7-16 Chapter 20 Table 20.3-1.
This is based on the observed density of the encountered glacial till, typical SPT values for the glacial
till, and the mapped stratigraphy. These conditions are assumed to be representative for the
subsurface across the site.
Design Parameters
The U.S. Geological Survey (USGS) completed probabilistic seismic hazard analyses (PSHA) for
the entire country in November 1996, which were updated and republished in 2002 and 2008. We
used the ATC Hazard by location website to estimate seismic design parameters at the site. Table 3,
below, summarizes the recommended design parameters.
TABLE 3:
2018 IBC PARAMETERS FOR DESIGN OF SEISMIC STRUCTURES
Spectral Response Acceleration (SRA) and Site Coefficients Short Period
Mapped SRA Ss = 1.317g
Site Coefficients (Site Class C) Fa = 1.200g
Maximum Considered Earthquake SRA SMS = 1.581g
Design SRA SDS = 1.051g
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Peak Ground Acceleration
The mapped peak ground acceleration (PGA) for this site is 0.557g. To account for site class,
the PGA is multiplied by a site amplification factor (FPGA) of 1.2. The resulting site modified peak
ground acceleration (PGAM) is 0.668g. In general, estimating seismic earth pressures (kh) by the
Mononobe-Okabe method are taken 30 to 50 percent of the PGAM, or 0.222g to 0.334g.
Seismic Hazards
Earthquake-induced geologic hazards may include liquefaction, lateral spreading, slope
instability, and ground surface fault rupture. Liquefaction is a phenomenon where there is a reduction
or complete loss of soil strength due to an increase in pore water pressure in soils. The increase in
pore water pressure is induced by seismic vibrations. Liquefaction primarily affects geologically
recent deposits of loose, uniformly graded, fine-grained sands and granular silts that are below the
groundwater table. Based on the dense nature of the glacially consolidated soils and the interpreted
depth of the groundwater table, it is our opinion that the risk to the development from liquefaction
or lateral spreading is low.
Foundation Support
Based on the subsurface conditions encountered at the locations explored, we recommend
that spread footings be founded on the dense glacial till soils. Footings can be founded on the
undocumented fill, provided that a field representative of GeoResources verifies the subsurface
conditions at time of the excavation of the footings. We recommend that in-situ density test be taken
at the base of foundation area that the test pits show to be fill. The fill should be compacted to at least
95 percent of the maximum dry density (MDD) as determined by the modified proctor. Tests should
also be taken at depths of 1 and 2 feet below the bottom of the footings.
The soil at the base of the excavations should be disturbed as little as possible. All loose, soft
or unsuitable material should be removed or compacted, as appropriate. A representative from our
firm should observe the foundation excavations to determine if suitable bearing surfaces have been
prepared, particularly if any areas of the foundation will be situated in existing fill material.
All footing elements should be embedded at least 18 inches below grade for frost protection.
Footings founded on the advance outwash can be designed using an allowable soil bearing capacity
of 2,000 psf (pounds per square foot) for combined dead and long-term live loads. The weight of the
footing and any overlying backfill may be neglected. The allowable bearing value may be increased
by one-third for transient loads such as those induced by seismic events or wind loads.
Lateral loads may be resisted by friction on the base of footings and floor slabs and as passive
pressure on the sides of footings. We recommend that an allowable coefficient of friction of 0.35 be
used to calculate friction between the concrete and the underlying soil. Passive pressure may be
determined using an allowable equivalent fluid density of 300 pcf (pounds per cubic foot). Factors of
safety have been applied to these values.
We estimate that settlements of footings des igned and constructed as recommended will be
less than ½ inch, for the anticipated load conditions, with differential settlements between comparably
loaded footings approaching total settlements. Most of the settlements should occur essentially as
loads are being applied. However, disturbance of the foundation bearing surfaces during
construction could result in larger settlements than predicted. We recommend that all foundations
be provided with footing drains.
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Floor Slab Support
We anticipate that the floor will consist of a slab-on-grade. Slab-on-grade floors should be
supported on the glacial on structural fill prepared as described in the “Structural Fill” section of the
report. Areas of significant organic debris and loose or disturbed soils should be removed.
We recommend that floor slabs be directly underlain by a minimum 4 inch thick layer of pea
gravel or washed 5/8-inch crushed rock that contains less than 2 percent fines as shown on the typical
wall drainage and backfill detail, Figure 6. This layer should be placed in a single lift and compacted
to an unyielding condition.
A synthetic vapor retarder is recommended to control moisture migration through the slabs.
This is of particular importance where moisture migration through the slab is an issue, such as where
adhesives are used to anchor carpet or tile to the slab.
A subgrade modulus of 200 pci (pounds per cubic inch) may be used for floor slab design. We
estimate that settlement of the floor slabs designed and constructed as recommended, will be 1/2
inch or less over a span of 50 feet.
Temporary Excavations
All job site safety issues and precautions are the responsibility of the contractor providing
services/work. The following cut/fill slope guidelines are provided for planning purposes only.
Temporary cut slopes will likely be necessary during grading operations or utility installation.
All excavations at the site associated with confined spaces, such as utility trenches and
retaining walls, must be completed in accordance with local, state, or federal requirements. Based on
current Washington Industrial Safety and Health Act (WISHA, WAC 296-155-66401) regulations, we
classify the upper undocumented fill as Type B soils and the lower undisturbed glacial till as Type A.
According to WISHA, for temporary excavations of less than 20 feet in depth, the side slopes
in Type B soils should be laid back at an inclination of 1H:1V or flatter from the toe to the top of the
slope. Type A soils exposed slope faces should be covered with a durable reinforced plastic
membrane, jute matting, or other erosion control mats during construction to prevent slope raveling
and rutting during periods of precipitation. These guidelines assume that all surface loads are kept
at a minimum distance of at least one half the depth of the cut away from the top of the slope and
that significant seepage is not present on the slope face. Flatter cut slopes will be necessary whe re
significant raveling or seepage occurs, or if construction materials will be stockpiled along the top of
the slope.
Where it is not feasible to slope the site soils back at these inclinations, a retaining structure
should be considered. Where retaining structures are greater than 4 feet in height (bottom of footing
to top of structure) or have slopes of greater than 15 percent above them, they should be engineered
per Washington Administrative Code (WAC 51-16-080 item 5). This information is provided solely for
the benefit of the owner and other design consultants, and should not be construed to imply that
GeoResources assumes responsibility for job site safety. It is understood that job site safety is the
sole responsibility of the project contractor.
Site Drainage
All ground surfaces, pavements and sidewalks at the site should be sloped to direct water
away from the structures and property lines. Surface water runoff should be controlled by a system
of curbs, berms, drainage swales, and or catch basins, and conveyed to an appropriate discharge
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point. We recommend that footing drains be installed for the structure in accordance with IBC 1805.4.2.
The roof drain should not be connected to the footing drain. If footings extend below the adjacent
municipal stormwater system, a sump and pump system may be required.
Stormwater Infiltration
Based on our site observations, knowledge of the 2016 King County Surface Water Design
Manual and our subsurface explorations, it is our opinion that onsite infiltration of the stormwater
generated by the proposed development is likely not feasible for this project.
Per Section 5.2.1 of the 2016 KCSWDM, stormwater runoff cannot be infiltrated into
undocumented fill. Additionally, impermeable glacial till underlies the undocumented fill. Therefore,
traditional onsite infiltration or dispersion of stormwater runoff generated by the proposed
development is likely not feasible for this project because of site constraints.
We recommend that stormwater runoff should be collected and conveyed to an appropriate
discharge point below the eastern slope, if feasible. Appropriate erosion and sediment control
measures should be included in the plans to limit the potential for erosion at the site.
EARTHWORK RECOMMENDATIONS
Site Preparation
While no topsoil was observed in our explorations, some vegetation will likely need to be
removed along with any other deleterious materials including existing structures, foundations, or
abandoned utility lines. Areas of thicker topsoil or organic debris may be encountered in areas of
heavy vegetation or depressions.
Where placement of fill material is required, the stripped/exposed subgrade areas should be
compacted to a firm and unyielding surface prior to placement of any fill. Excavations for debris
removal should be backfilled with structural fill compacted to the densities described in the
“Structural Fill” section of this report.
We recommend that a member of our staff evaluate the exposed subgrade conditions after
removal of vegetation and topsoil stripping is completed and prior to placement of structural fill. The
exposed subgrade soil should be proof-rolled with heavy rubber-tired equipment during dry weather
or probed with a 1/2-inch-diameter steel rod during wet weather conditions.
Soft, loose, or otherwise unsuitable areas delineated during proofrolling or probing should be
recompacted, if practical, or over-excavated and replaced with structural fill. The depth and extent of
overexcavation should be evaluated by our field representative at the time of construction. The areas
of old fill material should be evaluated during grading operations to determine if they need mitigation;
recompaction or removal.
Structural Fill
All material placed as fill associated with mass grading, as utility trench backfill, under building
areas, or under roadways should be placed as structural fill. Material placed for structural fill should
be free of debris, organic matter, trash and cobbles greater than 3 inches in diameter. The moisture
content of the fill material should be adjusted as necessary for proper compaction.
The suitability of material for use as structural fill will depend on the gradation and moisture
content of the soil. As the amount of fines (material passing US No. 200 sieve) increases, soil becomes
increasingly sensitive to small changes in moisture content and adequate compaction becomes more
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difficult to achieve. During wet weather, we recommend use of well-graded sand and gravel with less
than 5 percent (by weight) passing the US No. 200 sieve, such as Gravel Backfill for Walls (WSDOT 9-
03.12(2)). If prolonged dry weather prevails during the earthwork and foundation installation phase
of construction, higher fines content (up to 10 to 12 percent) may be acceptable.
The structural fill should be placed in horizontal lifts of appropriate thickness to allow
adequate and uniform compaction of each lift. Structural fill should be compacted to at least 95
percent of MDD (maximum dry density as determined in accordance with ASTM D1557).
The appropriate lift thickness will depend on the structural fill characteristics and compaction
equipment used. As a general rule, lift thicknesses should not exceed about 4- to 6-inches when using
hand operated equipment such as jumping jacks or plate compactors. Thicker lifts, up to about 12
inches, may be appropriate for larger equipment such as rollers or hoe-packs. We recommend that
our representative be present during site grading activities to observe and document the earthwork
activities and perform in-place field density tests.
Suitability of On-Site Materials as Fill
During dry weather construction, non-organic on-site soil may be considered for use as
structural fill; provided it meets the criteria described above in the “Structural Fill” section and can
be compacted as recommended. If the soil material is over -optimum in moisture content when
excavated, it will be necessary to aerate or dry the soil prior to placement as structural fill. We
generally did not observe the site soils to be excessively moist at the time of our subsurface
exploration program.
The native glacial till and the existing fill encountered at the locations explored generally
consisted of silty sandy gravel and poorly graded gravel with sand. These soils are generally
comparable to “common borrow” material (WSDOT Common Borrow 9-03.14(3)) and will be suitable
for use as structural fill provided the moisture content is maintained within 2 percent of the optimum
moisture level. Because of the high fines content, these soils are moisture sensitive, and will be
difficult to impossible to compact during wet weather conditions, or where seepage occurs. If these
soils are excessively moist to saturated, it will be necessary to aerate or dry the soil prior to placement
as structural fill.
We recommend that completed graded-areas be restricted from traffic or protected prior to
wet weather conditions. The graded areas may be protected by placing a layer of clean crushed rock
material containing less than 5 percent fin or a rat slab of CDF.
Erosion Control
Weathering, erosion and the resulting surficial sloughing and shallow land sliding are natural
processes. As noted, no evidence of surficial raveling or sloughing was observed at the site. To
manage and reduce the potential for these natural processes, we recommend erosion hazards be
mitigated by applying Best Management Practices (BMPs), as outlined in the 2016 KCSWDM. The
project civil engineer/designer, should prepare a drainage and temporary erosion control plan per the
KCSWDM showing which BMPs will be used.
Temporary erosion control BMPs should be installed at the site prior to the beginning of
clearing, grading, or other construction activities, and should be updated and maintained throughout
construction until final site stabilization is established. Temporary erosion control BMPs may include,
but are not limited to:
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• Silt fencing and appropriate soil stockpiling techniques to prevent silty stormwater
from leaving the site,
• Jute matting, hydroseeding, or plastic covering to protect exposed soils,
• Straw wattles, quarry spall armoring, check dams, or other energy attenuation BMPs
to slow the flow of stormwater over slopes and within drainage channels, and,
• Swales and berms to convey construction stormwater away from the slope.
Erosion protection measures should be in place prior to the start of grading activity on the
site. Where native vegetated is removed because of clearing and grading activities, a dense vegetative
groundcover, grass lawn, or native vegetation should be reestablished as soon as feasible. Permanent
erosion control, such as mulched landscaping areas, groundcovers, hardscaping, or grass lawns,
should be established as soon as feasible once final grades have been completed. All permanent
erosion control methods should be maintained after construction activities have been completed.
LIMITATIONS
We have prepared this report for Mr. Steve Davis and other members of the permitting and
design team for use in evaluating a portion of this project. Subsurface conditions described herein
are based on our observations of exposed soils on the parcel. This report may be made available to
regulatory agencies or others, but this report and conclusions should not be construed as a warranty
of subsurface conditions. Subsurface conditions can vary over short distances and can change with
time.
Variations in subsurface conditions are possible between the explorations and may also occur
with time. A contingency for unanticipated conditions should be included in the budget and
schedule. Sufficient monitoring, testing and consultation should be provided by our firm during
construction to confirm that the conditions encountered are consistent with those indicated by the
explorations, to provide recommendations for design changes should the conditions revealed during
the work differ from those anticipated, and to evaluate whether earthwork and foundation installation
activities comply with contract plans and specifications.
The scope of our services does not include services related to environmental remediation and
construction safety precautions. Our recommendations are not intended to direct the contractor's
methods, techniques, sequences or procedures, except as specifically described in our report for
consideration in design.
If there are any changes in the loads, grades, locations, configurations or type of facilities to be
constructed, the conclusions and recommendations presented in this report may not be fully
applicable. If such changes are made, we should be given the opportunity to review our
recommendations and provide written modifications or verifications, as appropriate.
◆ ◆ ◆
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We have appreciated the opportunity to be of service to you on this project. If you have any
questions or comments, please do not hesitate to call at your earliest convenience.
Respectfully submitted,
GeoResources, LLC
Davis Carlsen, GIT
Staff Geologist
Keith S. Schembs, LEG Eric W. Heller, PE,LG
Principal Senior Geotechnical Engineer
DC:KSS:EWH/dc
DocID: Davis.S348thSt.RG
Attachments: Figure 1: Site location Map
Figure 2: Site & Exploration Plan
Figure 3: Site Vicinity Map
Figure 4: NRCS Soils Map
Figure 5: Geologic Map
Figure 6: Typical Wall Drainage & Backfill Detail
Appendix A - Subsurface Explorations
Appendix B - Laboratory Test results
Approximate Site Location
Map created from King County iMap (https://gismaps.kingcounty.gov/iMap/)
Not to Scale
Site Location Map
Proposed Commercial Development
803 South 348th Street
Federal Way, Washington
PN: 2021049087
DocID: Davis.S348thSt.F August 2021 Figure 1
Notes:
Provided Site Plan
Not to Scale NORTH
Exploration number and approximate locations (GeoResources 2021)
Site & Exploration Map
Proposed Commercial Development
803 South 348th Street
Federal Way, Washington
PN: 2021049087
DocID: Davis.S348thSt.F August 2021 Figure 2
TP-1
TP-#
TP-2
TP-3
Approximate Site Location
Map created from King County iMap (https://gismaps.kingcounty.gov/iMap/)
Not to Scale
Site Vicinity Map
Proposed Commercial Development
803 South 348th Street
Federal Way, Washington
PN: 2021049087
DocID: Davis.S348thSt.F August 2021 Figure 3
Approximate Site Location
Map created from Web Soil Survey (http://websoilsurvey.sc.egov.usda.gov/App/WebSoilSurvey.aspx)
Soil
Type Soil Name Parent Material Slopes Erosion
Hazard
Hydrologic
Soils Group
EwC
Everett-Alderwood
gravelly sandy
loams
Basal till with some volcanic ash 6 to 15 Slight to
Moderate B
No Norma sandy loam Alluvium 0 to 2 Slight A/D
Sk Seattle Muck Grassy organic material 0 to 1 Slight B/D
Not to Scale
NRCS Soils Map
Proposed Commercial Development
803 South 348th Street
Federal Way, Washington
PN: 2021049087
DocID: Davis.S348thSt.F August 2021 Figure 4
Approximate Site Location
An excerpt from the Geologic Map of the Poverty Bay 7.5-Minute Quadrangle, Washington
by Derek B. Booth, Howard H. Waldron, and Kathy G. Troost (2003)
Qvr Recessional outwash deposits
Qvt Glacial Till
Not to Scale
Geologic Map
Proposed Commercial Development
803 South 348th Street
Federal Way, Washington
PN: 2021049087
DocID: Davis.S348thSt.F August 2021 Figure 5
Notes
Typical Wall Drainage and Backfill Detail
Proposed Commercial Development
803 South 348th Street
Federal Way, Washington
PN: 2021049087
DocID: Davis.S348thSt.F August 2021 Figure 6
1. Washed pea gravel/crushed rock beneath floor slab could be
hydraulically connected to perimeter/subdrain pipe. Use of 1”
diameter weep holes as shown is one applicable method. Crushed
gravel should consist of 3/4” minus. Washed pea gravel should consist
of 3/8” to No. 8 standard sieve.
2. Wall backfill should meet WSDOT Gravel Backfill for walls Specification
9-03-12(2).
3. Drainage sand and gravel backfill within 18” of wall should be
compacted with hand-operated equipment. Heavy equipment should
not be used for backfill, as such equipment operated near the wall
could increase lateral earth pressures and possibly damage the wall.
The table below presents the drainage sand and gravel gradation.
4. All wall back fill should be placed in layers not exceeding 4” loose
thickness for light equipment and 8” for heavy equipment and should
be densely compacted. Beneath paved or sidewalk areas, compact to
at least 95% Modified Proctor maximum density (ASTM: 01557-70
Method C). In landscaping areas, compact to 90% minimum.
5. Drainage sand and gravel may be replaced with a geocomposite core
sheet drain placed against the wall and connected to the subdrain
pipe. The geocomposite core sheet should have a minimum
transmissivity of 3.0 gallons/minute/foot when tested under a gradient
of 1.0 according to ASTM 04716.
6. The subdrain should consist of 4” diameter (minimum),
slotted or perforated plastic pipe meeting the requirements
of AASHTO M 304; 1/8-inch maximum slot width; 3/16- to 3/8-
inch perforated pipe holes in the lower half of pipe, with
lower third segment unperforated for water flow; tight joints;
sloped at a minimum of 6”/100’ to drain; cleanouts to be
provided at regular intervals.
7. Surround subdrain pipe with 8 inches (minimum) of washed
pea gravel (2” below pipe” or 5/8” minus clean crushed gravel.
Washed pea gravel to be graded from 3/8-inch to No.8
standard sieve.
8. See text for floor slab subgrade preparation.
Materials
Drainage Sand and Gravel ¾” Minus Crushed Gravel
Sieve Size % Passing by
Weight
Sieve Size % Passing by
Weight
¾” 100 ¾” 100
No 4 28 – 56 ½” 75 – 100
No 8 20 – 50 ¼” 0 – 25
No 50 3 – 12 No 100 0 – 2
No 100 0 – 2 (by wet
sieving)
(non-plastic)
Appendix A
Subsurface Explorations
SOIL CLASSIFICATION SYSTEM
MAJOR DIVISIONS
GROUP
SYMBOL
GROUP NAME
COARSE
GRAINED
SOILS
More than 50%
Retained on
No. 200 Sieve
GRAVEL
More than 50%
Of Coarse Fraction
Retained on
No. 4 Sieve
CLEAN
GRAVEL
GW
WELL-GRADED GRAVEL, FINE TO COARSE GRAVEL
GP
POORLY-GRADED GRAVEL
GRAVEL
WITH FINES
GM
SILTY GRAVEL
GC
CLAYEY GRAVEL
SAND
More than 50%
Of Coarse Fraction
Passes
No. 4 Sieve
CLEAN SAND
SW
WELL-GRADED SAND, FINE TO COARSE SAND
SP
POORLY-GRADED SAND
SAND
WITH FINES
SM
SILTY SAND
SC
CLAYEY SAND
FINE
GRAINED
SOILS
More than 50%
Passes
No. 200 Sieve
SILT AND CLAY
Liquid Limit
Less than 50
INORGANIC
ML
SILT
CL
CLAY
ORGANIC
OL
ORGANIC SILT, ORGANIC CLAY
SILT AND CLAY
Liquid Limit
50 or more
INORGANIC
MH
SILT OF HIGH PLASTICITY, ELASTIC SILT
CH
CLAY OF HIGH PLASTICITY, FAT CLAY
ORGANIC
OH
ORGANIC CLAY, ORGANIC SILT
HIGHLY ORGANIC SOILS
PT
PEAT
NOTES: SOIL MOISTURE MODIFIERS:
1. Field classification is based on visual examination of soil Dry- Absence of moisture, dry to the touch
in general accordance with ASTM D2488-90.
Moist- Damp, but no visible water
2. Soil classification using laboratory tests is based on
ASTM D2487-90. Wet- Visible free water or saturated, usually soil is
obtained from below water table
3. Description of soil density or consistency are based on
interpretation of blow count data, visual appearance of
soils, and or test data.
Unified Soils Classification System
Proposed Commercial Development
803 South 348th Street
Federal Way, Washington
PN: 2021049087
DocID: Davis.S348thSt.F August 2021 Figure A-1
Test Pit TP-1
Location: Northern portion of site
Approximate Elevation: 227’
Depth (ft) Soil Type Soil Description
0 - 5¼ GP Grey poorly graded GRAVEL with some coarse sand (dense, moist) (undocumented fill)
5¼ - 8¼ SM Grey-tan silty sandy GRAVEL (dense, moist) (glacial till)
Terminated at 8¼ feet below ground surface.
No caving observed at time of excavation
No groundwater seepage observed.
Mottling observed at approximately 5¼ feet below ground surface
Test Pit TP-2
Location: Central portion of site
Approximate Elevation: 226’
Depth (ft) Soil Type Soil Description
0 - 7½ GP Grey poorly graded GRAVEL with some coarse sand (dense, moist) (undocumented fill)
7½ - 8 SM Grey-tan silty sandy GRAVEL (dense, moist) (glacial till)
Terminated at 8 feet below ground surface.
No caving observed at time of excavation
No groundwater seepage observed.
Mottling observed at approximately 7½ feet below ground surface
Logged by: DC Excavated on: August 3, 2021
Test Pit Logs
Proposed Commercial Development
803 South 348th Street
Federal Way, Washington
PN: 2021049087
DocID: Davis.S348thSt.F August 2021 Figure A-2
Test Pit TP-3
Location: Southern portion of site
Approximate Elevation: 225’
Depth (ft) Soil Type Soil Description
0 - 7 GP Grey poorly graded GRAVEL with some coarse sand (dense, moist) (undocumented fill)
7 - 8 SM Grey-tan silty sandy GRAVEL (dense, moist) (glacial till)
Terminated at 8 feet below ground surface.
No caving observed at time of excavation
No groundwater seepage observed.
Mottling observed at approximately 7 feet below ground surface
Logged by: DC Excavated on: August 3, 2021
Test Pit Logs
Proposed Commercial Development
803 South 348th Street
Federal Way, Washington
PN: 2021049087
DocID: Davis.S348thSt.F August 2021 Figure A-3
Appendix B
Laboratory Test Results
These results are for the exclusive use of the client for whom they were obtained. They apply only to the samples tested and are not indicitive of apparently identical samples.Tested By: Checked By:
Particle Size Distribution Report
PERCENT FINER0
10
20
30
40
50
60
70
80
90
100
GRAIN SIZE - mm.
0.0010.010.1110100
% +3"Coarse
% Gravel
Fine Coarse Medium
% Sand
Fine Silt
% Fines
Clay
0.0 35.7 30.5 4.2 5.5 9.8 14.36 in.3 in.2 in.1½ in.1 in.¾ in.½ in.3/8 in.#4#10#20#30#40#60#100#140#200Test Results (ASTM D 6913 & ASTM D 1140)
Opening Percent Spec.*Pass?
Size Finer (Percent)(X=Fail)
Material Description
Atterberg Limits (ASTM D 4318)
Classification
Coefficients
Date Received:Date Tested:
Tested By:
Checked By:
Title:
Date Sampled:Location: TP-1, S-1
Sample Number: 102356 Depth: 7'
Client:
Project:
Project No:Figure
Silty Sandy GRAVEL (GM)
2.0
1.5
1.25
1
.75
.5
0.375
#4
#10
#20
#40
#60
#100
#200
100.0
91.0
83.6
69.7
64.3
46.7
41.7
33.8
29.6
26.8
24.1
20.8
17.7
14.3
NP NV NP
GM A-1-a
36.9309 32.5855 16.8936
13.7780 2.2605 0.0869
Natural Moisture: 4.0%
7/30/21 8/17/21
MAW
KSS
PM
7/30/21
Steve Davis
Proposed Commerical Development
Davis.S348thPlSW
PL=LL=PI=
USCS (D 2487)=AASHTO (M 145)=
D90=D85=D60=
D50=D30=D15=
D10=Cu=Cc=
Remarks
*(no specification provided)
GeoResources, LLC
Fife, WA B-1
